By default when you insert an image into blogger you get the following code
<img border="0" data-original-height="200" data-original-width="600" height="200" src="https://3.bp.blogspot.com/abcde.png" width="600" />
You have the change the code into following. You have to copy paste the image src. Obtain the height from the data-original-height and data-original-width tags.
<amp-img src="" alt="A responsive image" height="200" width="600" layout="responsive"></amp-img>Do not forget to include the layout="responsive" attribute otherwise the image will not be responsive.
Below is an example image loaded from unsplash. You can resize the screen and see that the image is responsive.